/Engine/Plugins/Online/OnlineSubsystemUtils/Source/OnlineSubsystemUtils/Public/PartyBeaconClient.h
Tell the server to cancel a pending or existing reservation
[UFUNCTION](Programming/UnrealArchitecture/Reference/Functions)(Server, Reliable, WithValidation) virtual void ServerCancelReservationRequest ( const [FUniqueNetIdRepl](API\Runtime\Engine\GameFramework\FUniqueNetIdRepl) & PartyLeader )